Manufactured objects often have to conform to dimensional and/or geometrical tolerances. These may be specified by the manufacturer or ultimate user of the object e.g. to ensure that the object behaves correctly in service. Thus it is common to subject objects to post-manufacture inspection procedures.
A known technique for determining the dimensional accuracy of objects involves driving a coordinate measuring probe or stylus over the surface of the object. Such a probe typically comprises a stiff elongate member (formed e.g. of carbon fibre) having a small hard (typically industrial ruby) sphere at the measuring end. An example of a commercially available probe is the Renishaw SP600™ analogue scanning probe. In one arrangement, the probe is fitted to a Renishaw MIH™ head providing angular movement about two orthogonal axes in 7.5 increments. The head is then mounted to the quill of a programmable coordinate measuring machine (CMM), which controls the movement of the probe. A typical CMM comprises a quill providing z-direction movement, a carriage assembly which carries the quill and provides x- and y-direction movement, and a measuring table, as shown for example in U.S. Pat. No. 5,189,805. The CMM typically also comprises a programmable computer to control the probe movements. CMMs are commercially available from a number of suppliers.
Sensing means associated with the probe detects when and where the sphere contacts a surface and the CMM translates that information into a spatial coordinate for the surface contact point with the sphere. Thus by making contact with the object at spaced apart points along a programmed guide path, the probe can provide discrete measurements of the relative positions of these points. In this way, shape characteristics (such as profiles or outlines) of the actual object can be built up, and these characteristics can then be compared for conformity with e.g. predetermined tolerance limits.
Conventionally, the operator programs a guide path by a teach-and-learn technique in which he drives the probe under manual control to specific points along the desired guide path, and saves the positions of those points in the memory of the CMM. When performing a measurement run, the CMM subsequently drives the probe from point-to-point in an order determined by the operator, taking measurements along the way. In defining the guide path, the operator will also specify approach directions and probe angles to ensure that the probe or quill does not collide with the object. As and when necessary, the operator may also program the quill to change the probe (usually at a probe change station remote from the object) so that e.g. a probe with a longer elongate member is used to reach otherwise-inaccessible parts of the object.
Gas turbine engine components, and particularly critical components such as blades, vanes and discs, are commonly inspected using such techniques.
A problem can arise, however, when the object under inspection has internal surfaces (which are inaccessible to the probe. For example, it is common for gas turbine blades and vanes to be formed as hollow components, with internal cavities which reduce the weight of the component and/or provide cooling channels. In particular, a known technique for producing hollow titanium alloy fan blades involves superplastic forming of the blade to expand internal blade cavities, followed by acid dipping of the blade to taper etch the external surface and provide the desired aerofoil shape. The external surface may then be linished to give the desired surface finish. However, as a result of these procedures, variability can be introduced into the thickness of the blade walls which define the cavities, i.e. there can be a degree of uncertainty in the positions of the internal surfaces of the cavities.